Cellular senescence is conditions under which a cell that should originally carry out cell growth cannot perform cell division in response to growth stimulation. A senescent cell is characterized by its large flat nucleus and a cellular senescence-specific β-galactosidase (senescence-associated β-galactosidase; SA-β-galactosidase) activity. In addition to morphological changes, the cellular senescent cell (senescent cell) shows characteristic changes in the gene expression patterns and the like.
There are many causes which trigger cellular senescence. For example, human normal fibroblast is a cell most generally used in the studies on cellular senescence. It is known that cellular senescence is induced when this is exposed to ionizing radiation or cultured under a high oxygen partial pressure. Also, in recent years, it has been reported that cellular senescence is rapidly induced after about 1 week when an activated oncognic RAS is expressed by force in human normal fibroblast.
However, the cellular senescence most thoroughly studied is a cellular senescence by the replicative life span (telomere-dependent) in which a normal cell reaches an aging cell after a finite number of cell divisions.
Since the cellular senescence generated by these different causes finally shows the same phenotype, it is considered that it occurs by the same molecular mechanism, but its details are not clear.
The cellular senescence based on the replicative life span is a phenomenon in which a normal cell becomes senescent cell after a certain number of cell divisions which vary depending on the kind of the cell. In the case of the most frequently used human normal fibroblast, a fetal cell shows an aging phenotype after about 60 to 80 times of cell division. This phenomenon was reported by Hayflick in 1962. However, it has been unclear for a long time that intracellular mechanism of the recording of the number of cell divisions. In recent years, it has been shown that DNA replication of the chromosomal terminal telomere is not complete due to the so-called “end replication problem”, and the telomere length is shortened from about 50 to 200 base pairs per cell division, and that a cellular senescence is induced when this shortening reaches its threshold value. However, it is not clear that in what manner the cellular senescence occurs by the shortening of telomere length and whether or not its molecular mechanism is identical to cellular senescence caused by other causes.
On the other hand, MAPK (Mitogen-activated protein kinase) pathway is a cascade of protein kinases which exist in the cytoplasm and are activated by the intracellular and extracellular stimuli and thereby phosphorylate and activate downstream inactive protein kinases. The final target kinase MAPK phosphorylates and activates a specific transcription factor, and the cell responds to stimuli through the induction of the expression of a group of genes by the transcription factor.
The upstream kinase which phosphorylates the final target kinase MAPK is called MAPKK (MAPK2K), and the upstream kinase which phosphorylates the aforementioned MAPKK is called MAPKKK (MAPK3K). Two or more of the MAPK pathway are present, and MAPK, MAPKK and MAPKK are known as each of them.
The classical MAPK pathway which was discovered and analyzed earliest in the history comprises Raf-Mek-Erk, and it responds to mainly extracellular stimuli, such as growth factor and the like and is activated via Ras. PD98059 is known as a specific inhibitor of Mek.
On the other hand, JNK (c-Jun N-terminal kinase) and p38 protein are known as the MAPK which is induced by cytokine of tumor necrosis factor (TNF) and the like that induce stress and apoptosis. Also, the MAPKK and MAPKKK of JNK and p38 protein are not single, but two or more of them are respectively known.
SB203580 is known as a compound which specifically inhibits the function of the p38 protein, and it is known that this compound inhibits production of inflammatory cytokines interleukin-1, interleukin-6, interleukin-8 or TNF and therefore is useful a therapeutic agent of inflammatory diseases (WO 97/33883).